THE principle on which this novel instrument is constructed to act, consists in obtaining the weight of the column of water resting on the ground, a principle which, simple as it is, and beautiful in its simplicity, could only have been discovered and applied by the light of science, as the principle of lineal measurement first was by the light of nature.

As there may be some of our readers who are unacquainted with the subject, we will here shortly explain that it is a property common to all fluids to exert an equal pressure in every direction, the amount of which pressure increases in proportion to the depth below the surface, and which is in fact the weight of the superincumbent mass; thus on every part of the earth's surface is borne the weight or pressure of the whole atmosphere lying above it, which amounts at the level of the sea to about 15 Ibs. on every square inch; and as this pressure is exerted in every direction it presses equally upon all objects, whether upwards, downwards, or laterally. So again, at any given depth below the surface of water, there is exerted in every direction a pressure equal to the column of water lying upon it up to the surface, added to the weight of the column of air resting on it there; thus at 1000 feet deep in the sea any substance there immersed in it would be exposed over every square inch of its surface to the weight of a column of water 1000 feet high, or 12,000 cubic inches, and weighing about 446 Ibs., added to the weight of a corresponding column of air equal to 15 Ibs. more. The above is the property of water which Mr. BEKTHON has availed himself of; we will now endeavour to explain the manner in which he has got over the difficulty of applying it, and point out its advantages over the ordinary method of ascertaining the depth of water from a vessel in motion by lineal measurement.

A lead and line are still used by Mr.

BERTHON, but they are each different in character from those commonly in use. The lead is a hollow oblong, having openings in it at the sides to admit the water from without, and one on the top to allow the line to pass into it. The peculiarity of the line is, that it is a hollow one, in fact it is a small flexible tube, the one we have seen is made of gutta percha; but as that substance would not answer in hot climates, or possibly in a very cold one, they will in future be made of another material which will secure the same object, which will be as strong as rope of similar size, and which is not affected by any change of temperature to which the atmosphere or waters are subject in any part of the globe. To one end of this line a waterproof bag is attached, which bag is inserted within the hollow lead above described, the latter being then hermetically secured to the* line at its point of junction with the bag. We have now secured a bag inflated with air, which by means of the tubular line will instantaneously transmit any pressure on it to the opposite extremity of the line, however long it may be; thus, if a similar bag but uninflated were herme- o tically attached to that end, the two bags might alternately be filled and emptied by pressure on the one or the other. Bxvt suppose that at the opposite end to the lead instead of a bag the tube be inserted into a small instrument somewhat similar to a barometer, and that any pressure from without upon the inflated bag be communicated through the tubular line to the surface of a small reservoir of mercury, which shall thereby be forced up a glass tube having a graduated index to it, showing the amount of pressure upon the surface of the mercury.

The apparatus is in fact then complete.

The leadsman throws his lead in the ordinary manner; as it sinks the pressure is increased on the bag and the mercury rises in the meter, showing every instant by the index the number of feet or fathoms that the lead has descended, and ceasing to rise the moment the latter has reached the bottom.

Having now explained as clearly as we are able the principle and mode of application of Mr. BEETHON'S invention, we will endeavour to show what we conceive to be its advantages as compared with the ordinary method of sounding. The disadvantages of the ordinary method are— 1st. The difficulty, when a vessel is going fast through the water, of getting a perpendicular sounding at all; either the lead will not reach the ground before it passes astern of the person heaving it, or he may not be able to gather in the slack line until it has done so. It is, however, immaterial whether Mr. BEETHON'S lead be perpendicular to the leadsman or not, and if it does not arrive at the ground until considerably astern of him, the index will still show the perpendicular depth of water. Accordingly, soundings may be obtained when going at the most rapid rate through the water; and Mr.

BERTHON has himself, with a very small lead, obtained them correctly in 5 or 6 fathoms water, from a steamer when going at the rate of 14 miles an hour.

2nd. By the ordinary method all correctness must depend on the skill of the leadsman, and how often do we see in a man-of-war one leadsman after another called from his post, and their places supplied by others, in the hope of at last rinding one who should get the soundings quickly and correctly. Indeed, very few men, comparatively speaking, ever make really accurate and skilful leadsmen. But if this difficulty be experienced in a man-of-war, how much more likely is it to be so in an indifferently manned merchant vessel. The application of Mr. BERTHON'S method, however, requires no such skill, nothing more being required of the leadsman than to throw the lead forward into the water in the usual manner.

3rd. The obtaining correct soundings in the night-time, when objects on the land which would indicate the position of the ship are obscured or invisible, is even of more importance than by day; but as the marks on the ordinary lead-line which define the various depths are of different colours, they are liable to be mistaken the one for the other, in the dark. By Mr. BERTHON'S method, a continuation of the tubular line is conveyed on board to the meter, which may be put up in any convenient and conspicuous position, to which a light might be attached in the night-time, and the soundings called out, if necessary, by an officer or other person appointed to read them off the index, instead of their being sung, often very indistinctly, by the leadsman himself.

It is not proposed by the inventor that this sounding instrument should supersede the use of the ordinary lead and line, but be used in conjunction with it, so that one would be a check on the other, whilst soundings could be obtained by his method under circumstances when they could not be by the usual mode.

Amongst other advantages of BERTHON'S lead, besides that of its instantaneously registering the depth, are the following:— At very slow speeds, as, for instance, when a steamer feels her way into port in a fog, moving at the rate of two or three knots per hour, this lead, hove over from the bow or jib-boom, may be dragged along the bottom, actually under the ship's keel, and thus keep up a continual register of depth.

Again, in Surveying, the lead may be towed along the bottom by a boat going slowly, and the indicator being held in the hand, every undulation of the ground is correctly indicated at every instant, and a sunken .rock or stone cannot escape detection.

As a correct and most convenient tidegauge it is unrivalled. It is necessary only to heave over the lead from a vessel at anchor, and leave it with plenty of stray line on the bottom ; and, as its indications come from the bottom, and not from the surface, it is as correct in a sea-way as in smooth water: the pressure upon any part of the bottom being quite independent of the undulations of the surface of the water.

There is a collateral application of this principle, with which we will .conclude these remarks. It is one of considerable importance and utility. A tube-line with air-bag, such as we have described, is placed in a ship, so that the air-bag remaining in the lowest part of the hold, and the- indicator fixed in some conspicuous place, as, for instance, the fore-side of the binnacle; by this arrangement a leak is instantly detected, and the quantity of water in the hold always correctly shown..